High temperature solid-state proton conductors (HT-SSPCs) have drawn much attention since the discovery of fast proton conduction in the acceptor-doped perovskite-type (ABO3≈CaTiO3) structure alkaline or rare-earth containing complex metal oxides by Iwahara and others (Iwahara et al., 1981; Uchida et al., 1983; Norby, 1999; Kreuer, 1996; Iwahara, 1995; Iwahara, 1996; Bonanos, 2001). They find potential applications in various ceramic protonic devices that include gas sensors, proton exchange membrane fuel cells (PEMFCs), H2 pumps, H2 separation and steam electrolyser (Kreuer, 1996; Coors, 2003; Zhang et al., 2003; Proton Conductors: Solids, Membranes and Gels—Materials and Devices, 1992). Accordingly, a large numbers of perovskite and perovskite-related structure metal oxides have been investigated for fast proton conduction (Iwahara et al., 1990; Kreuer et al., 1994; Glöckner et al., 1999; Kruth et al., 2007). Among the various HT-SSPCs studied, hitherto, aliovalent metal ion-doped, e.g., BaCe1−xYxO3-δ (BCY) (Iwahara, 1995) and B-site non-stoichiometric double perovskite-type Ba3Ce1+xNb2-xO9-δ (BCN) (Nowick and Du, 1995; Nowick et al., 1999) exhibit high proton conductivity at elevated temperatures in H2O containing atmospheres. FIG. 6a shows the crystal structure of the parent single perovskite (ABO3≡SrTiO3), and FIG. 6b shows an ordered double perovskite (A2BB′O6≡Ba2CaWO6).
The chemical stability of electrolyte with electrode materials and costs are major concerns in commercialization fuel cells, including solid oxide fuel cells (SOFCs) and proton membrane fuel cells (PEMFCs), and there is always a need of good chemically stable against reaction with electrolyte and cheap materials. Currently, some of the members of La1−xSrxMnO3 (LSM) has being commonly employed as the cathode material for SOFCs because of its high catalytic activity for oxygen reduction, matching thermal expansion coefficient, and chemical compatibility with yttria stabilized zirconia (YSZ) (Yamamoto et al., 1987). While LSM has shown promising performance at temperatures above 800° C., its performance decreases rapidly as the operating temperature decreases. Substitution of Sr for RE in RECoO3 (RE=rare earth; Sm for SSC) was found to be better catalyst for O2 reduction compared LSM, and exhibits low overpotentials for oxygen reduction reaction (ORR) (Hyang et al., 1996; Hammouche et al., 1989; Xia et al., 2002; Ishihara et al., 1998). Pt and Pt-metal alloys remain as potential electrodes (anode and cathode) for both low temperature polymer membrane and high temperature ceramic membrane based PEMFCs. However, currently very few mixed ionic and electronic conducting ceramic cathodes were investigated for high temperature PEMFCs.